Native Image Committer and Community Meeting 2024-08-22

[christianwimmer]

List of all past and upcoming meetings: #3933

New and Noteworthy

• Native Image Layers:
  [GR-53663] Implement Native Image Layers options. #9385
  [GR-55085] Add support for object replacers referring to prior layer constants. #9288
  [GR-55523] Change ClassLoaderFeature to be layer-aware. #9347
  [GR-54137] Persist and reload analysis graph in extension layers #9422
  [GR-54946] Refactor prior layer method logic. #9497
  [GR-55538] Garbage collection with multiple layers. #9542
  [GR-57235] Persist types and methods flags across layers #9537

• GuestGraal
  [GR-49426] GuestGraal. #9301
  [GR-55686] GuestGraal: Add EE support. #9515

• Use cgroup support derived from HotSpot VM
  [GR-51479] Implement cgroup support in native code. #8989
  [GR-53451] Eagerly initialize libsvm_container #9484
  [GR-55305] Add namespace to libcontainer. #9477
  [GR-54128] Re-import libsvm_container code and add change detection annotations #9250

• Special exceptions for missing configurations:
  [GR-54685] Condition typeReached for Unsafe.allocateInstance. #9392

• Usability:
  [GR-57384] Preserve origin why resources are included in an image. #9546

• Cleanups:
  [GR-54918] Rename JDK23OrLater to JDKLatest and remove obsolete predicates #9211
  [GR-51000] Always use a heap-base register. #9432 Disabling support for isolates is unsupported, but we do not want to remove the code completely for now. This PR reduces the differences between running with isolates and without isolates, reducing maintenance costs.
  [GR-55094] Remove PolyglotNativeAPI. #9407

• Monitoring / tools:
  [GR-57310] NMT simplified virtual memory tracking. #9505
  [GR-54994] Avoid duplicate JFR ThreadSleep events. #9291

Deep Dive: GuestGraal

Goal: Build libgraal with a native image tool that uses a different version of the compiler.

Two different versions of the Graal compiler within one image build

• "Hosted Graal": The compiler used by native image for static analysis and AOT compiler
• "Guest Graal": The compiler built into libgraal for later JIT compilation in the HotSpot VM. We also require the full JDK where libgraal is going to be integrated in.

The Graal compiler is only useful as a native image (called "libgraal"), because using normal Java class loading (called "jargraal") pollutes the HotSpot heap and metadata with the compiler. To better decouple compiler development and native image development for OpenJDK project Galahad , it is helpful if the Graal compiler in the OpenJDK repository can be built into libgraal by using native image as an external build tool (like gcc).
From the point of view of OpenJDK, Native Image is then a build tool like gcc, i.e., there is only a binary dependency and not a source code dependency on Native Image.

Implementation approach:

• GuestGraal is loaded in a separate class loader that has only the JDK system class loader as the parent (not the image generator, not even JVMCI)
• Graal requires several initializations at image build time. Those initializations are triggered from a Feature (that currently must be in Native Image because only there the Feature API is available), but then reflectively invoke the actual initializations in GuestGraal
• Snippets must be parsed at image build time, but using GuestGraal and the JVMCI from GuestGraal. Therefore, a new HotSpot processes is spawned (using the HotSpot VM from GuestGraal) that parses the snippets and serializes them into a textual format.
• There are a few substitutions for the compiler still remaining. They still are in Native Image for now.

Future work:

• Support for Truffle JIT compilation is still under development, and needs to be finished before GuestGraal can be the default and only way to build libgraal.
• Move more code like the Feature and the substitutions into the compiler project. For this, the compiler itself needs to depend on the Native Image API.